1. Field of Invention
This invention pertains to the catalytic hydrogenation of aldoses such as glucose to produce glycerol and other polyols. It pertains more particularly to a multiple-stage process wherein a portion of the sugars produced from the second or alditol (sorbitol) hydrogenolysis stage is passed to a third hydrogenation stage for further conversion to glycerol.
2. Description of Prior Art
It has been noted that the catalytic hydrogenolysis of alditols such as sorbitol in a fixed bed reactor using nickel catalyst to produce polyols such as glycerol and glycols also forms some polyglycerols and aldoses (glucose) from the feed of alditols (sorbitol), respectively. Dehydrogenation of alcohols to sugars and dehydration of glycerol to polyglycerols presents the problem of build up of such substances in the recovery step because of the usual recycle stream to the sorbitol cracking reaction zone, which not only contains unconverted sorbitol and alditols but also the sugars and polyglycerols which are neither removed in the distillation step nor extracted out by the solvent in an extraction step, if used. To prevent such buildup of sugars and polyglycerols which is detrimental to process operations, withdrawal of a purge stream has been used from the distillation separation step. However, such a purge stream presents a loss of valuable products and reactants along with the sugars from the process, thereby decreasing yields and increasing costs. Thus, it is desirable to recover the sugars, polyglycerols and unconverted sorbitol by some appropriate reprocessing steps.
A disclosure regarding hydrogenolysis of sorbitol is provided by Clark in Industrial & Engineering Chemistry, Vol. 50, No. 8 (Aug. 1958), page 1125. Aqueous solution containing 40% of 99% D-sorbitol were used with 1% calcium hydroxide promotor and 50% nickel on kieselguhr catalyst suspended in a slurry with the feed in a stirred reactor. Conditions used were 2000-5600 psi hydrogen partial pressure, 215.degree.-245.degree. C. (419.degree.-473.degree. F.) temperature and reaction times up to 400 minutes (6.7 hrs) to produce glycerol, ethylene glycol, propylene glycol, and other more minor products.
U.S. Pat. No. 2,965,679 to Conradin discloses a similar process for producing glycerol and glycols from sugar alcohols using a suspended nickel on kieselguhr catalyst in an autoclave type reactor. Reaction conditions are 200.degree.-300.degree. C. temperature, 500-1000 atmospheres pressure and pH of 8-10, followed by filtration to remove catalyst and separation of the products.
Van Ling et al disclosed in Journal of Applied Chemisty, Vol 19, pages 43-45, hydrogenation experiments using slurried catalyst in autoclave reactor on feeds of sucrose, glucose and fructose in methanol-water solution to produce glycerol. Catalyst used was CuO-CeO.sub.2 -SiO.sub.2 with 0-5% Ca(OH).sub.2 addition to the feed. Reaction conditions used were 200.degree.-250.degree. C. temperature, 100-300 atmospheres pressure and 10-120 minutes reaction time.
U.S. Pat. No. 3,471,580 to Hellwig et al discloses that by using a single or multi-stage upflow ebullated bed catalytic reactor at 200.degree.-550.degree. F. temperature and 700-3500 psia hydrogen partial pressure, glycerol and glycols can be produced from saccharides. Examples of the conditions used for converting a sorbitol type feed to glycerol in a single stage reaction were about 375.degree. F. temperature, 1700 psia hydrogen partial pressure, 1.2 liquid hourly space velocity (LHSV), and using nickel on alumina catalyst to produce roughly 50 W % glycerol and 20 W % ethylene glycol and propylene glycol, with the remainder being methanol, ethanol, isopropanol, and other products (col. 5, lines 40-53).
It is believed that none of these known processes are presently being used commercially to produce glycerol and related products on a continuous basis. Thus, further process improvements in conversion of aldoses and alditols are desired not only for achieving continuous operations, reduced reaction conditions and increased glycerol product yields, particularly using improved catalysts in fixed-bed reactors, but to prevent loss of valuable sugars and glycols which are usually removed from the distillation step by a purge stream.